1. Field of the Invention
The present invention relates to concrete sewers that are prone to corrosion and more particularly to concrete sewers that are prone to sulfide corrosion.
2. Description of the Prior Art
It has been found that a two step biological process corrodes collection system infrastructures, including concrete sewers and manholes. This is known as "sulfide corrosion", and is increasingly deteriorating today's public works infrastructure.
Sulphide corrosion has detrimental effects on the collection system infrastructure ranging from more frequent odor complaints or potentially lethal atmospheres to failure of collection system pipes and manholes.
In the first step, usually occurring in slow moving sewage below the water line where anaerobic conditions can exist, sulfur reducing anaerobic bacteria, primarily belonging to the genus Desulfovibrio, reduce sulfate ions to sulfide ions. In addition, sulfide can be produced by the bacterial decomposition of protein, and through the decomposition of other organosulfur compounds. However, it is generally recognized that the predominant mechanism for sulfide generation in sewer collection systems is sulfate reduction. Through chemical equilibria, some of the sulfide ions form hydrogen sulfide gas molecules and escape out of the liquid sewage into the headspace atmosphere of the sewer pipe.
In the second step, a different group of sulfur bacteria, primarily belonging to the genus Thiobacillus, establish colonies in the concrete pipe, and through an oxidation process, convert the atmospheric hydrogen sulfide to sulfuric acid with the liberation of free protons and a drop of Ph. The resulting acid attacks the concrete, causing the ultimate destruction of the pipe. It is believed that the acid reacts with the lime in the concrete converting it into a soft putty-like gypsum.